The invention arose during further development efforts regarding marine trolling motors as shown in U.S. Pat. Nos. 4,009,677, 4,099,478 and 4,305,012, incorporated herein by reference.
In the art of marine trolling propulsion, a small electric driven outboard motor unit is mounted to the bow or transom of a boat and employed for slow speed maneuverability and positioning for fishing. The electric drive motor is housed within a lower submerged housing unit and coupled to drive a suitable trolling propeller. A separate high power internal combustion outboard motor is normally provided for rapid propulsion of the boat to the fishing location, after which the trolling motor is employed. For trolling, thrust from the trolling propeller is first transferred from the propeller to the electric trolling motor housing and then to the boat.
Electric trolling motors are often designed to operate from a conventional 12 volt battery. Higher thrust units operating from a 24 volt supply are available, but require two 12 volt batteries. A direct drive connection of the electric motor to the propeller provides a simple and reliable trolling motor with minimal cost. However, direct drive is subject to an inherent trade-off in efficiency because the motor has an optimum operating speed which is higher than that of the trolling propeller.
The above noted patents show planetary gear reduction systems for driving a large diameter low rpm trolling propeller from a higher rpm electric trolling motor. The large slower turning trolling propeller is more efficient in producing thrust at low boat speeds than high speed propellers. The larger trolling propeller requires more torque to turn it, which torque is provided by the gear reduction between the motor and the propeller.
In the present invention, a marine trolling propulsion system is provided with a traction drive speed reducer between the motor and propeller. Gears and gear teeth noise are eliminated, and replaced by rolling contact.
The invention is particularly advantageous in various design features which facilitate low manufacturing cost. The design inherently accommodates tolerance deviations in those members providing the loading of the rolling contact members. There is particular tolerance of axial deviations affecting radial loading of traction rollers. There is also tolerance in various members for movement transverse to the axis of rotation.
The design enables different axes of rotation of the propeller shaft and the electric motor drive shaft such that such shafts do not have to be exactly aligned. The design enables a floating intermediate shaft which self-seats and tracks the rollers, without being constrained to align with either the input drive shaft from the electric motor or the propeller shaft. Inner race structure is keyed to the floating intermediates shaft and is designed to rock about such shaft transversely to the axis of rotation to equalize contact loading and to enable the inner race structure to self-seat and take its alignment from the rollers. The elimination of the alignment requirements of the rotating shafts eliminates concerns of binding and the strict manufacturing tolerances otherwise required to prevent same, including the attendant high machining costs, etc.
The design provides overload slip protection enabling the electric motor drive shaft to keep rotating even if the propeller is momentarily stopped, e.g. upon striking a stump, whereby to avoid mechanical breakage within the drive. In addition, uniform loading of the normal contact force on the rollers is provided by applying substantially constant axial force versus axial deflection. This reduces manufacturing cost, provides increased and uniform life by avoiding overly loaded rollers, provides uniform overload slip capability, and accommodates mechanical wear without a reduction of the radial loading of the rollers.
In the preferred embodiment, planet cage structure locates off a three roller ball pattern. The planet cage structure is also designed for limited rocking about a point on the propeller shaft center to ensure equal division of roller contact loading from propeller thrust, and to transmit propeller thrust through the traction rollers and outer race to the housing structure without a separate thrust bearing. In combination with above noted rocking inner race structure, this further provides uniform and equally distributed roller contact engagement.